1. Field of the Invention
The present invention relates to an aqueous resin composition useful as a coating on various substrates such as wood, metals, and molded resin products; a binder for paints and printing inks; a coating material; and an adhesive. The present invention also relates to an aqueous resin composition which, when dried, can form a tough and highly water resistant film.
2. Description of the Related Art
In the field of coating formation, a switch from emulsions in organic solvents to aqueous emulsions is strongly desired from the points of view of environmental hygiene and savings in resources. However, aqueous emulsions are not fully satisfactory in the water resistance, toughness, and adhesive strength of the resulting coating. A common practice for improving these properties is to introduce a functional group into an aqueous emulsion so that a crosslinked coating is formed. As an aqueous emulsion for formation of the crosslinked coating, a one-pack emulsion comprising a mixture of a curing agent and a polymer is demanded from the aspects of applicability and workability.
To fulfill this demand, U.S. Pat. No. 3,345,336 (Japanese Patent Publication No. 20053/71) discloses the crosslinking of a carbonyl group-containing polymer emulsion with a polyhydrazide compound. An aqueous emulsion used in this method is a one-pack emulsion. After the emulsion is applied for coating, water in the emulsion dries, whereupon the hydrazide groups and the carbonyl groups crosslink as a result of dehydration condensation to form a film. The polyhydrazide compound used here refers to a compound having at least two or more hydrazide groups. Examples of this compound are dihydrazides of an aromatic dicarboxylic acid such as phthalic acid, isophthalic acid, or terephthalic acid. Preferred examples are dihydrazides of an aliphatic saturated dicarboxylic acid having 2 to 10 carbon atoms, such as oxalic acid or sebacic acid. Particularly preferred examples are dihydrazides of adipic acid.
Various improvements have subsequently been made in one-pack crosslinkable aqueous emulsions containing polyhydrazides. Such improvements are described in Deutsche Offenlegungschrift 2819092 (Japanese Unexamined Patent Publication No. 144432/79), Deutsche Offenlegungschrift 2803258 (Japanese Unexamined Patent Publication No. 110248/79), Japanese Unexamined Patent Publication No. 3850/82, Japanese Unexamined Patent Publication No. 3857/82, Japanese Unexamined Patent Publication No. 96643/83, Japanese Unexamined Patent Publication No. 104902/83, Japanese Unexamined Patent Publication No. 38468/85, Japanese Unexamined Patent Publication No. 25163/87, Japanese Unexamined Patent Publication No. 62853/87, Deutsche Offenlegungschrift 3720860 (Japanese Unexamined Patent Publication No. 20256/89), Japanese Unexamined Patent Publication No. 234416/89, Japanese Unexamined Patent Publication No. 68669/91, Japanese Unexamined Patent Publication No. 227947/92 (Deutsche Offenlegungschrift 4016056), Japanese Unexamined Patent Publication No. 249587/92, and WO 96/01252.
Most of these one-pack crosslinkable aqueous emulsions use dihydrazides of aliphatic dicarboxylic acids as polyhydrazide compounds, and often use adipic acid dihydrazide preferably. Adipic acid dihydrazide functions as an effective crosslinking agent, but its solubility in water is insufficient, resulting in insufficient crosslinking. Consequently, the resulting coating is not fully water resistant.
One-pack crosslinkable aqueous emulsions using polyhydrazide compounds having three or more functional groups are also exemplified in some of the previously quoted prior art publications. Japanese Patent Publication No. 6861/86 describes a tri- or tetrahydrazide of pyromellitic acid, and a tri- or tetrahydrazide of nitrilotriacetic acid. Japanese Unexamined Patent Publication No. 38468/85, Deutsche Offenlegungschrift 3720860, and Deutsche Offenlegungschrift 4016056 describe pentane-1,3,5-tricarboxylic acid trihydrazides, hexene-4-1,2,6-tricarboxylic acid trihydrazides, and 3-cyanopentane-1,3,5-tricarboxylic acid trihydrazides.
The use of a hydrazide compound with three or more functional groups can increase the crosslinking density, and is thus expected to enhance the water resistance of a crosslinked coating. However, this does not mean that the higher the functionality of the hydrazide compound, the better outcome is obtained. An excessively high functionality means more hydrazide groups which cannot take part in crosslinking for reasons such as steric hindrance. As a result, the water resistance of the coating declines. Japanese Unexamined Patent Publication No. 234416/89 describes the use of a polyhydrazide formed by polymerizing two or more hydrazine compounds. When such a polyhydrazide is used, this polymeric hydrazide is slightly soluble in water, and cannot be fully dispersed in an aqueous emulsion. This poses difficulty in performing sufficient crosslinking. Japanese Unexamined Patent Publication No. 80619/94 describes a 1,2,3,4-butanetetracarboxylic acid hydrazide as an adsorbent of formaldehyde. This hydrazide is also sparingly soluble in water, and a coating of an emulsion formed by using it as a curing agent is poor in water resistance.
As described above, the use of a conventional aqueous resin composition containing a curing agent and a carbonyl compound still has posed difficulty in forming a crosslinked film having excellent properties such as water resistance and adhesion to a substrate.
It is an object of the present invention to provide a novel aqueous resin composition which has resolved the problems with the earlier technologies. That is, the object of this invention is to provide an aqueous resin composition containing fully dispersed components, and having high storage stability. It is another object of the present invention to provide an aqueous resin composition which, when dried, can form a film excellent in film intactness, transparency, whitening after water immersion, and adhesion after water immersion.